1. Field of the Invention
The present invention relates to an ink jet printer, and, more particularly, to a head-to-head alignment method and sensor for an ink jet printer.
2. Description of the Related Art
Many inkjet printers contain two printheads mounted to the same carrier. For example, one printhead can be monochrome only and the other printhead can be color only. Both printheads can be used on the same printed image. The monochrome printhead provides the saturated black and the color printhead provides all other colors. The dots fired by the two heads must be precisely aligned, horizontally and vertically, or else print quality defects will be seen. For example, the black and color dots will overlap and unprinted white areas will remain.
Vertical alignment errors cause vertical offsets between horizontal lines printed by each printhead. Horizontal alignment errors cause horizontal offsets between vertical lines printed by each printhead.
Many printers to date include a manual method of performing horizontal and vertical head-to-head alignment. Usually, this includes the printer driver printing a test page which includes a continuum of alignment possibilities, and having the user manually type-in at their personal computer a number or letter representing the pattern having the best alignment. From this input, the driver saves timing offsets to allow horizontal head-to-head alignment. Vertical alignment is achieved by moving the printed swath vertically within a printhead. A small percentage of the printhead nozzles are unused to allow the swath to be moved vertically.
What is needed in the art is an automatic, rather than manual, head-to-head alignment process, which removes the burden from the user.
The present invention provides a simple, low-cost, head-to-head alignment sensor and a simple, automatic head-to-head alignment method.
The invention comprises, in one form thereof, a printhead alignment sensor for an ink jet printer. At least two terminals define a gap therebetween. An electrical measuring device detects a change in an electrical resistance between two of the terminals when ink is in the gap between the at least two terminals.
The invention comprises, in another form thereof, a method of horizontally aligning a first printhead and a second printhead in an ink jet printer. A substrate having a target area with a width approximately equal to a width of an ink drop is provided. A carrier of the first printhead is moved from a first location toward the target area. A plurality of aligned first ink drops are jetted from the first printhead when the carrier of the first printhead is at a first jetting location. The aligned first ink drops are substantially parallel to the target area. It is sensed whether at least one of the first ink drops has been jetted onto the target area. The carrier of the first printhead is returned to the first location. The moving, jetting, sensing and returning steps are repeated until at least one of the first ink drops has been jetted onto the target area. The jetting steps are performed at various first jetting locations. A first reference location of the carrier of the first printhead is recorded. The first reference location is a location of the carrier of the first printhead when it is sensed that at least one of the first ink drops has been jetted onto the target area. A carrier of the second printhead is moved from a second location toward the target area. A plurality of aligned second ink drops are jetted from the second printhead when the carrier of the second printhead is at a second jetting location. The aligned second ink drops are substantially parallel to the target area. It is sensed whether at least one of the second ink drops has been jetted onto the target area. The carrier of the second printhead is returned to the second location. The moving, jetting, sensing and returning steps are repeated until at least one of the second ink drops has been jetted onto the target area. The jetting steps are performed at various second jetting locations. A second reference location of the carrier of the second printhead is recorded. The second reference location is a location of the carrier of the second printhead when it is sensed that at least one of the second ink drops has been jetted onto the target area. At least one offset is calculated based upon the first reference location and the second reference location.
An advantage of the present invention is that printhead-to-printhead alignment can be performed automatically, rather than manually. That is, alignment can be performed without printing a test page. No user interaction is required. The alignment may take place automatically as soon as a new printhead is identified as having been installed.
Another advantage is that the method allows high accuracy of alignment at little cost. The sensing circuit requires just a few low cost components. Also, the cost of the sensor is much less than that of a reflective, optical type sensor.
Yet another advantage is that only a rough alignment of the sensor in the printer is required for ease of printer manufacturing assembly.